You didn't mention what engine, I just spent 7 hours on a spark plug challenge on my 01 Ram with 5.9 Magnum.

without too many gory details, got the shield off, with a dremel and carbide burr , then the hex was rounded, used a tool from Grip tite, that promptly snapped off the all the spark plug except the threads that remained in the engine.
Started the engine and the ceramic blew out and I got an easy out to remove the threaded portion..

Yes,
I soaked it for 3 days in PB and Kroil... no help.

Still have 2 more to do...